Pre-ignition in a flame propagation (or “spark-ignition” as the terms will be used interchangeably throughout) engine describes an event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events. Many passenger car manufacturers have observed intermittent pre-ignition in their production turbocharged gasoline engines, particularly at low speeds and at medium-to-high loads. At these elevated loads, pre-ignition usually results in severe engine knock and loss of performance.
It is believed the auto-ignition of oil droplets and/or fuel-oil mixture droplets that accumulate in the piston top land area are one of the leading causes for this low-speed pre-ignition phenomenon. It is also believed that small amounts of oil may be transferred from below the oil control ring to the piston top land area due to unusual piston ring movement. At low speeds, in-cylinder pressure dynamics (compression and firing pressures) are somewhat different at high load conditions than they are at lower loads due to strongly retarded combustion phasing and high boost as well as peak compression pressures which can influence ring motion dynamics. Other possible sources of pre-ignition are believed to be soot deposits accumulating inside the combustion chamber and localized air/fuel mixture auto-ignition.
Pre-ignition can sharply increase combustion chamber temperatures and lead to rough engine operation or loss of performance. Traditional methods of eliminating pre-ignition are available and include proper spark plug selection, proper fuel/air mixture adjustment, and periodic cleaning of the combustion chambers. Such methods, however, do not attempt to predict the occurrence of pre-ignition. Given that most modern day automotive engines are equipped with onboard computerized engine management systems, a means of detecting the conditions leading up to a pre-ignition event would permit the management system to adjust one or more engine control parameters in order to mitigate an upcoming pre-ignition cycle.
Therefore, a way a determining when conditions are favorable for the occurrence of a pre-ignition event in a modern day spark ignition engine would be advantageous and allow the engine management system to take steps to prevent or mitigate the event before it occurs.